This invention relates to an abrader for use in the mirror polishing of glass articles such as optical glasses, to a method for shaping the grinding surface of said abrader and to a method for effecting the mirror polishing of glass by use of said abrader.
Optical glasses with mirror surfaces such as lenses for use in cameras, spectacles and microscopes, prisms and filters are today manufactured by a series of operations comprising roughing, fine lapping (smoothing) and mirror polishing operations.
The roughing operation constitutes a step wherein the glass surface is shaped with a grindstone or the glass surface is roughed up by moving a lapping board on the glass surface while keeping a granular lapping powder and a lapping liquid supplied to the interface therebetween.
The fine polishing operation is a step in which the glass surface is treated by interposing between the lapping board and the glass surface a lapping powder having a fine particle diameter of the order of #2000 to #500 Japanese Industrial Standard (average diameter 8 .mu.m to 34 .mu.m) which is finer than that of the lapping powder used in the roughing operation.
In a modified form, this fine polishing operation may be carried out by directly grinding the glass surface with diamond pellets formed by blending diamond powder and a binder.
The mirror polishing operation is for finishing the glass surface to perfect mirror smoothness by grinding the glass surface with the lapping board while continuously supplying a suspension of cerium oxide, zirconium oxide or red iron oxide to the interface between the lapping board and the glass article.
As the lapping board for use in the operations described above, there is used the type of lapping board which is formed by attaching a sheet of pitch, wax, woolen cloth or polyurethane resin to the surface of a disc made of cast iron.
According to the generally accepted theory, while the shaping of the glass surface in the roughing and fine polishing operations is mainly accomplished by causing fine glass fracture in the surface region, the shaping of the glass surface to the perfect mirror smoothness in the mirror polishing operation is attained by the fine cutting of the glass surface owing to the interaction among the glass surface, the lapping board and the lapping powder such as cerium oxide, zirconium oxide or red ion oxide, coupled with the elimination of bulges and dents on the surface due to heat flow in the glass surface, chemical reaction, etc.
For the mirror polishing of glass surface to be effected advantageously by means of the lapping board, it is imperative that the following requirements be fulfilled:
(1) The glass surface should be pressed with a uniform magnitude of pressure by the lapping board. PA0 (2) The lapping powder should be uniformly distributed throughout the entire interface between the glass article and the disc abrader. PA0 (3) The particle size of the lapping powder and the concentration of lapping powder in the suspension should both be proper for the operation.
It is no easy matter, however, to fulfill all these requirements. Further, since this mirror polishing operation necessitates use of suspension, the work is messy. As the lapping board is operated by an abrading machine, the suspension is likely to cause difficulties in the maintenance of the entire abrasion system.
What is more, it frequently happens in this operation that fine particles of the lapping powder from the suspension collect on the surface of a glass article under treatment and continue to cling thereto even after termination of said mirror polishing operation. For removal of the clinging fine particles, ultrasonic washing or even manual work involving use of a sharp blade is often resorted to.
The suspension is used repeatedly in this operation. As the number of repetitions increases, the lapping powder in the suspension undergoes gradual size reduction through friction and consequently the abrading capacity thereof dwindles by degrees. Eventually, it becomes necessary to start using a fresh lapping powder, in which case the new lapping powder tends to inflict scratches on the glass surface. In fact, rejects occur mostly because of such defects.
The inventors experimentally manufactured an abrader by blending a lapping powder with a polyvinyl acetal resin and molding the resultant blend and a lapping plate incorporating diamond dust and tried them in actual operations. However, both proved to be deficient in abrading capacity, durability, polishing accuracy, etc. and therefore unacceptable for practical use.
U.S. Pat. No. 3,915,671 which involves as one of the inventors thereof the inventor of the present invention covers a method for the manufacture of a porous, resin-bonded grinding tool. The grinding tool manufactured by this method comprises a cured unsaturated polyester resin and an abrasive material. Examples of the abrasive material usable in the manufacture of said grinding tool include fused alumina, silicon carbide, diamond dust, emery, garnet and glass powder.
With the grinding tool which contains such abrasive material, mirror polishing of the grade proper to optical glasses cannot be accomplished.
The present invention also embraces a method for shaping the grinding surface of an abrader.
This method is intended for uniformizing the pressure with which the glass surface being abraded and the lapping board are kept in contact with each other and also for ensuring uniform distribution of the suspension which is supplied continuously to the interface during the polishing operation, whereby the accuracy of polishing will be heightened.
Where a pitch plate is used as the lapping board, for example, the shaping of the grinding surface is effected by employing a method which takes advantage of the thermoplasticity of pitch and which comprises pressing the pitch plate against the surface of the standard plate in water heated to 40.degree. to 70.degree. C and allowing the plate in that state to cool off gradually. In the case of a lapping board in which a sheet of polyurethane is attached to the disc surface, the shaping of the grinding surface is effected by a method which comprises fastening this sheet by the medium of an adhesive agent to the surface of the disc while keeping the sheet pressed against the surface of the standard pate and thereafter grinding the surface of the attached sheet and the surface of the standard plate against each other. Since this sheet of polyurethane is very resistant to wear, it may be necessary on some occasions to continue said mutual grinding for a long time (on the order of several hours). The operation of surface shaping the lapping board prior to the polishing of the glass surface consumes a fairly long time no matter which method is employed. And this operation must be repeated at frequent intervals, because the grinding surface of the abrader is gradualy deformed as the polishing is continued.
Such being the case, need has long been felt for simplifying the method of surface shaping and reducing the time required for surface shaping.
The primary object of this invention is to provide an abrader for use in the mirror polishing of glass, which abrader permits the mirror polishing operation to be carried out with ease and convenience and produces a polished surface of high accuracy.
Another object of this invention is to provide a method for shaping the grinding surface of said abrader for use in the mirror polishing of glass.
Still another object of this invention is to provide a method for effecting the mirror polishing of glass by use of said abrader.